Ipamorelin and CJC‑1295: growth hormone secretagogues compared

Growth hormone is released from the pituitary gland in pulses, under the control of two main signals from the brain and gut. Ipamorelin and CJC-1295 are synthetic peptides that each amplify one of those signals. They are frequently discussed together, but they differ in receptor, structure, duration of action and the depth of evidence behind them. This analysis compares the two.

Two routes to growth hormone release

The pituitary responds to growth hormone-releasing hormone (GHRH) from the hypothalamus, acting at the GHRH receptor. It also responds to a second class of signal through the growth hormone secretagogue receptor. For years this receptor was known only through synthetic compounds that activated it. In 1999, researchers identified its natural ligand: ghrelin, a 28-amino-acid peptide from the stomach carrying an octanoyl group that is essential for its activity^[1].

CJC-1295 is an analogue of GHRH. Ipamorelin acts at the ghrelin receptor. They therefore reach the same outcome, greater growth hormone release, through separate receptor systems.

Ipamorelin: a selective ghrelin receptor agonist

Ipamorelin is a pentapeptide (Aib-His-D-2-Nal-D-Phe-Lys-NH₂) developed in a medicinal chemistry programme at Novo Nordisk in the 1990s^[2]. Its defining feature is selectivity. Earlier growth hormone-releasing peptides, such as GHRP-6 and GHRP-2, also raised the stress hormones ACTH and cortisol. In swine, ipamorelin did not raise ACTH or cortisol above the levels seen with GHRH, even at doses more than 200 times higher than those needed for growth hormone release. The authors described it as the first agonist of this receptor with a selectivity for growth hormone similar to that of GHRH^[2].

Human pharmacology

A dose-escalation study in healthy men characterised ipamorelin's behaviour after intravenous infusion. Its terminal half-life was about two hours. Each dose produced a single episode of growth hormone release, peaking at around 40 minutes, followed by a decline to negligible levels^[3]. Ipamorelin therefore produces a brief pulse rather than sustained elevation.

Animal and clinical findings

In adult female rats treated for 15 days, ipamorelin increased longitudinal bone growth in a dose-dependent manner without changing total IGF-I levels or markers of bone turnover^[4].

Ipamorelin reached a phase 2 clinical trial, not for growth but for post-operative ileus, the temporary paralysis of the gut after abdominal surgery, because ghrelin receptor activation has pro-motility effects. Among 114 patients analysed after bowel resection, ipamorelin was well tolerated, but it did not significantly shorten the time to tolerating a solid meal compared with placebo (median 25.3 versus 32.6 hours; p = 0.15)^[5].

CJC-1295: a long-acting GHRH analogue

The therapeutic use of natural GHRH has been limited by its short duration of action^[6]. CJC-1295 is a modified GHRH(1–29) with four amino-acid substitutions and a reactive maleimide group attached to a lysine. After injection, this group forms a covalent bond with a cysteine residue on circulating albumin^[7]. In rats, CJC-1295 was detectable in plasma beyond 72 hours and produced a four-fold greater growth hormone response than unmodified GHRH(1–29) over two hours^[7].

Human pharmacology

In two randomised, placebo-controlled, double-blind ascending-dose studies in healthy adults aged 21 to 61, a single subcutaneous injection of CJC-1295 raised mean growth hormone concentrations two- to ten-fold for six days or more, and IGF-I concentrations 1.5- to three-fold for nine to eleven days. The estimated half-life was 5.8 to 8.1 days, and no serious adverse reactions were reported^[6].

A key question was whether constant stimulation would flatten the natural pulsatile pattern of growth hormone release. In healthy young men sampled every 20 minutes overnight, one week after a single injection, the frequency and size of growth hormone pulses were unchanged. Baseline (trough) levels rose 7.5-fold, and mean growth hormone and IGF-I levels each rose by about 45%^[8].

Animal findings

In mice lacking the GHRH gene, once-daily CJC-1295 normalised body weight and length, while dosing every two or three days was less effective. Treatment also increased pituitary growth hormone mRNA, suggesting proliferation of the hormone-producing cells^[9].

Side by side

Reading the pharmacokinetic contrast

The difference in time course is the most important practical distinction between the two peptides, and it is easy to misread.

Ipamorelin's two-hour half-life and single growth hormone peak mean that its effect resembles one additional pulse layered onto the body's own rhythm^[3]. CJC-1295 does something quite different. Anchored to albumin, it keeps stimulating the GHRH receptor for days. Its most striking effect in healthy men was a large rise in baseline growth hormone between pulses, while the pulses themselves were unchanged^[8].

These two patterns cannot be treated as equivalent simply because both raise average growth hormone or IGF-I. Pulsatility is thought to matter for many of growth hormone's physiological effects^[8], and a persistently raised baseline is a different physiological state from a short, discrete peak. The mouse data point the same way: CJC-1295 given every day normalised growth in GHRH-deficient animals, while the same dose given every two or three days did not^[9]. How often a secretagogue acts, not only how strongly, appears to shape the outcome.

The studies were also designed differently. The ipamorelin work used intravenous infusion in healthy men^[3], whereas the CJC-1295 studies used subcutaneous injection^[6]. Cross-study comparisons of peak concentrations or potency should therefore be treated with caution.

Why they are discussed together

Because the two peptides act at different receptors, it is often argued that combining them could produce a greater or more natural growth hormone response than either alone. That reasoning follows from the physiology. The sources reviewed for this article, however, do not include a controlled human study of the specific ipamorelin and CJC-1295 combination, so the claim remains untested here.

Safety considerations

A review of growth hormone secretagogues in humans concluded that they are generally well tolerated, and that they promote pulsatile release that remains subject to the body's own negative feedback. It also noted concern about rises in blood glucose through reduced insulin sensitivity, and the absence of long-term safety data, including on cancer incidence and mortality^[10].

Summary

Ipamorelin and CJC-1295 illustrate two distinct strategies for increasing growth hormone release: a short-acting, highly selective ghrelin receptor agonist, and a long-acting GHRH analogue anchored to albumin. Both are pharmacologically well characterised in early human studies. Neither has an extensive clinical efficacy record in the published literature reviewed here, and ipamorelin's one phase 2 trial did not meet its main efficacy endpoint. Claims about their combined use go beyond the available controlled evidence.